Gas separation is important in many industries and can typically be accomplished by flowing a mixture of gases over an adsorbent that preferentially adsorbs one or more gas components while not adsorbing one or more other gas components. The non-adsorbed components are then recovered as a separate product.
An important type of gas separation technology is swing adsorption, such as temperature swing adsorption (TSA) or pressure swing adsorption (PSA). PSA processes rely on the phenomenon of gases being more readily adsorbed within the pore structure or free volume of an adsorbent material when the gas is under pressure, i.e., the higher the gas pressure, the greater the amount readily-adsorbed gas adsorbed. When the pressure is reduced, the adsorbed component is released, or desorbed.
PSA processes may be used to separate gases of a gas mixture because different gases tend to fill the micropore of the adsorbent to different extents. If a gas mixture, such as natural gas, is passed under pressure through a vessel containing a polymeric or microporous adsorbent that is more selective towards carbon dioxide than it is for methane, at least a portion of the carbon dioxide is selectively adsorbed by the adsorbent, and the gas exiting the vessel is enriched in methane. When the adsorbent reaches the end of its capacity to adsorb carbon dioxide, it is regenerated by reducing the pressure, thereby releasing the adsorbed carbon dioxide. The adsorbent is then typically purged and repressurized and ready for another adsorption cycle.
TSA processes rely on the phenomenon that gases at lower temperatures are more readily adsorbed within the pore structure or free volume of an adsorbent material compared to higher temperatures, i.e., when the temperature of the adsorbent is increased, the adsorbed gas is released, or desorbed. By cyclically swinging the temperature of an adsorbent bed, TSA processes can be used to separate gases in a mixture when used with an adsorbent that is selective for one or more of the components of a gas mixture.
U.S. Pat. No. 9,120,049, which is incorporated herein by reference, describes a rotary valve arrangement for a cyclical swing adsorption process using a single fixed bed, the single bed being sandwiched between a pair of rotors and a pair of stators. The fixed plate stators communicate with the adsorbent bed when an aperture in the associated rotor aligns with the corresponding aperture in the stator.
U.S. Pat. No. 9,162,175, which is incorporated herein by reference, describes a plurality of concentric adsorbent beds within a single vessel, each adsorbent bed having an equal face cross-sectional area.
U.S. Provisional Application No. 62/370,881, which is incorporated herein by reference, describes a tapered adsorbent bed wherein the cross-sectional area of the bed tapers in the direction of feed flow. It has been reported that tapered adsorbent beds can improve product purity in PSA cycling applications. See James A. Ritter et al., Tapered Pressure Swing Adsorption Columns for Simultaneous Air Purification and Solvent Vapor Recovery, 37 IND. ENG. CHEM. RES. 2783-91 (1998). Ritter reports, among other things, that in an adiabatic bed in PSA cycling, product purity is several orders of magnitude higher in a tapered bed compared to a non-tapered bed. Ritter, at 2787, FIG. 5. This improvement in purity is of particular significance to hydrogen production where very high purities, e.g. 99.9% purity or greater are required. Moreover, Ritter describes another advantage of tapered versus conventional adsorbent beds with respect to temperature profile across the bed. Ritter, at 2787, FIG. 6. For tapered beds, the temperature profile at the end of the feed or adsorption step is substantially lower in the mass transfer zone. This lower temperature maintains a higher CO2 adsorption as well as a higher water gas shift conversion in the case of a syngas feedstream. Tapered adsorbent beds, however, are irregularly shaped and cannot be easily incorporated into a tight refinery environment, where space is at a premium.
There remains a need in the industry for apparatus, methods, and systems are more efficient and that can be constructed and employed on a smaller footprint than conventional equipment. Compact designs are critical when the swing adsorption apparatus is to be deployed in remote locations, such as off-shore production platforms, arctic environments, or desert environments.